This invention relates to a method of controlled chemical vapor deposition (CVD) of a metal oxide ceramic layer, and more particularly, to additives for stabilization and efficiency enhancement of CVD processes for forming bismuth oxide ceramics, such as in the form of thin layers or films on semiconductor substrates for use in fabricating microelectronic semiconductor devices, for example, ferroelectric capacitors, and the like.
In fabricating microelectronic semiconductor devices and the like on a wafer substrate or chip, such as of silicon, to form an integrated circuit (IC), etc., various metal layers and insulation layers are deposited in selective sequence. To maximize integration of device components in the available substrate area to fit more components in the same area, increased IC miniaturization is utilized. Reduced pitch dimensions are needed for denser packing of components per present day very large scale integration (VLSI), e.g., at sub-micron (below 1 micron, i.e., 1,000 nanometer or 10,000 angstrom) dimensions.
Recently, ceramic phases with bismuth oxide (bismuth trioxide, Bi2O3) as a component have found widespread interest for application in semiconductor memories. In this regard, bismuth titanate (Bi4Ti3O12), strontium bismuth tantalate (SBT, SrBi2Ta2O9) and strontium bismuth titanate (SrBi4Ti4O15) possess ferroelectric properties, which make these ceramic materials interesting for applications in non-volatile memories, e.g., FeRAMs (ferroelectric random access memories).
Ferroelectric materials, such as strontium bismuth tantalate (SBT, SrBi2Ta2O9), also called an Aurivillius phase ferroelectric or mixed bismuth oxide layer structure, exhibit electric polarization in the absence of an externally applied electric field, such that the direction of polarization may be reversed by an electric field. Thus, ferroelectric capacitors are able to change their direction of polarization under an applied electric field, for instance, to switch between a xe2x80x9c1xe2x80x9d or a xe2x80x9c0xe2x80x9dvalue or state as might be required in a given IC system.
Very thin layers or films of ferroelectric ceramic materials are required for the above noted purposes, which materials, in addition, show a high conformality to the surface structure of the substrate. Moreover, it is necessary to produce these films rapidly and inexpensively. In practice, only a CVD process can meet all of these requirements.
Some examples of the fabrication of ferroelectric devices are shown in the following prior art.
U.S. Pat. No. 5,478,610 (Desu et al.), issued Dec. 26, 1995 (Desu Patent I), and its continuation in part U.S. Pat. No. 5,527,567, issued Jun. 18, 1996 (Desu Patent II), disclose a method of fabricating a layered structure oxide ferroelectric thin film by CVD involving chemical reaction between volatile organo metal compounds, such as alkyls, alkoxides, xcex2-diketonates or metallocenes of the metal elements to be deposited, and gases such as oxygen, to produce a non-volatile solid that deposits on a substrate. For example, after such a film is deposited on a substrate having a thin surface layer of a metal, e.g., Pt, and then ferroannealed, a second thin layer of the metal is deposited, followed by a second ferroanneal, for forming a ferroelectric capacitor in which the first metal layer is the bottom electrode, the second metal layer is the top electrode and the deposited ceramic oxide film is the dielectric separating the two metal layers.
The Desu Patents I and II contemplate depositing a SrBi2(TaxNb2xe2x88x92x)O9 or BaBi2(TaxNb2xe2x88x92x)O9 film on a substrate by pulsed laser deposition (PLD). This involves using as preferred precursors Ba(thd)2 (Ba-tetramethyl heptadione), Sr(thd)2 (Sr-tetramethyl heptadione), Bi(thd)3 (Bi-tetramethyl heptadione), Ta(OC2H5)5 (Ta-ethoxide) and Nb(OC2H5)5 (Nb-ethoxide) in stoichiometric ratios in a solvent that is an 8:2:1 mixture in moles of tetrahydrofuran (C4H8O), isopropanol (C4H10O) and tetraglyme (C10H22O5), while the substrate is heated to a high temperature of at least 450xc2x0 C. as claimed.
The Desu Patent II also contemplates depositing a strontium bismuth tantalate (SBT, SrBi2Ta2O9) film on a substrate, by a liquid source delivery (LSD) method. This involves using Sr(thd)2 (Sr-tetramethyl heptadione), Bi(C6H5)3 (Bi-triphenyl, BiPh3) and Ta(OC2H5)5 (Ta-ethoxide) as precursors in such 8:2:1 molar ratio solvent mixture, at a high temperature of 450-800xc2x0 C., in a two-step method, in which the film is first deposited at 450-600xc2x0 C. for 5 minutes, and then at 600-700xc2x0 C. for 30-120 minutes, compared to a one-step method wherein the film is deposited at 650xc2x0 C.
It is desirable to have a one-step method of forming a metal oxide ceramic layer on a substrate by CVD that can be conducted, inexpensively, rapidly and efficiently under vacuum pressure, especially at a comparatively low deposition temperature, with facilitated thermal decomposition of the precursor organo metal compound to its metal oxide having the same oxidation state as in the precursor compound, and with control of the in situ oxidation state of the deposited metal and the amount of oxygen in the formed layer, while suppressing the formation of volatile intermediates and of vacancies in the formed layer.
The foregoing drawbacks are obviated in accordance with the present invention by providing a protonating additive substance and/or an activating agent for facilitating a one-step method of forming a metal oxide ceramic layer on a substrate by CVD that can be conducted inexpensively, rapidly and efficiently under vacuum pressure, especially at a comparatively low temperature.
According to a first aspect of the invention, a method of forming a metal oxide ceramic layer on a substrate by CVD is provided, comprising conducting a gaseous flow of a vaporized solution of a precursor organo metal compound in a volatile organic solvent, e.g., plus an oxidizing gas, in the presence of a protonating additive substance in gaseous state, into contact with a surface portion of the substrate under a vacuum pressure at a thermal decomposition temperature effective for converting the precursor compound to its corresponding metal oxide, e.g., having the same oxidation state as in the precursor compound.
The additive substance is present in an amount sufficient for facilitating the thermal decomposition of the precursor compound and for controlling the in situ oxidation state of the deposited metal and the amount of oxygen in the formed layer, e.g., while suppressing the formation of volatile intermediates and of vacancies in the formed layer.
The additive substance may be water, a volatile carboxylic acid compound, a volatile ketone, a volatile amine or ammonia. The carboxylic acid compound may be a volatile carboxylic acid, a volatile carboxylic acid anhydride, a volatile carboxylic acid ester, a volatile carboxylic acid nitrile, a volatile carboxylic acid isonitrile or a volatile carboxylic acid aldehyde. The additive substance is desirably present in an amount of about 0.01-200 mols per mol of the precursor compound.
According to an alternative feature of the invention, the conducting of the gaseous flow is effected in the additional presence of an activating agent in gaseous state in an amount sufficient for producing in situ hydrogen-active compounds for enhancing the converting of the corresponding precursor compound to its metal oxide. The activating agent may be carbon monoxide, hydrogen or a volatile lower aliphatic hydrocarbon having 1-4 carbon atoms or aromatic hydrocarbon, which volatile hydrocarbon is optionally substituted by a bromo, iodo or nitro (xe2x80x94NO2) substituent, i.e., so as to provide a volatile brominated lower aliphatic or aromatic hydrocarbon, a volatile iodinated lower aliphatic or aromatic hydrocarbon or a volatile nitro group-containing lower aliphatic or aromatic hydrocarbon.
The decomposition temperature may be about 150-800xc2x0 C., preferably about 150-350xc2x0 C. or about 300-500xc2x0 C., and particularly below 450xc2x0 C., such as at least about 150xc2x0 C. or 200xc2x0 C. and at most about 445xc2x0 C. or 440xc2x0 C. or 430xc2x0 C. or 425xc2x0 C. or 400xc2x0 C. The vacuum pressure may be about 0.01-100 Torr, preferably about 0.5-20 Torr.
The substrate may be made of silicon, a hydrocarbon based polymer, or the like. Where the substrate is provided of silicon containing a surface layer of a metal, e.g., Pt, the decomposition temperature is desirably about 300-500xc2x0 C. and the vacuum pressure about 0.01-100 Torr, preferably about 0.5-20 Torr. Similarly, where the substrate is provided of a hydrocarbon based polymer containing a surface layer of a metal, the decomposition temperature is desirably about 150-350xc2x0 C. and the vacuum pressure about 0.01-100 Torr, preferably about 0.5-20 Torr.
Typically, the oxidizing gas is an oxygen-containing inert carrier gas, e.g., comprising about 20-80%, preferably 40-80%, oxygen. Alternatively, the oxidizing gas may be a nitrogen oxide. The vaporized solution of the precursor compound and the additive substance are each advantageously provided in a separate inert carrier gas.
Specifically, the precursor compound may be an organo bismuth compound such as a triorgano bismuth compound, and typically comprises a mixture of an organo bismuth compound, an organo strontium compound and an organo tantalum compound such as a mixture of a triorgano bismuth compound, a diorgano strontium compound and a pentaorgano tantalum compound. In particular, the precursor compound comprises a mixture of bismuth tris-2,2,6,6-tetramethyl heptane-3,5-dionate, designated Bi(thd)3, or bismuth triphenyl, designated BiPh3, plus strontium bis-2,2,6,6-tetramethyl heptane-3,5-dionate, designated Sr(thd)2, and tantalum 2,2,6,6-tetramethyl heptane-3,5-dionate tetra isopropoxide, designated Ta(OiPr)4(thd), such as in a molar ratio of 0.25:0.15:0.40, respectively.
Alternatively, the strontium bis-2,2,6,6-tetramethyl heptane-3,5-dionate (Sr(thd)2) may be provided in the form of the corresponding tetraglyme (2,5,8,11,14-pentaoxa pentadecane, CH3(CH2CH2O)4CH3) adduct or pentamethyl diethylene triamine (PA, i.e., N,N,Nxe2x80x2,Nxe2x80x3,Nxe2x80x3-pentamethyl diethylene triamine, (CH3)2Nxe2x80x94CH2CH2xe2x80x94N(CH3)xe2x80x94CH2CH2xe2x80x94N(CH3)2) adduct.
Typically, the mixture is provided in a solvent system of tetrahydrofuran/isopropyl alcohol/tetraglyme such as in a molar ratio of 8:2:1, respectively, or in a solvent system of octane/decane/pentamethyl diethylene triamine such as in a molar ratio of 5:4:1, respectively.
In particular, the mixture may be provided, e.g., in a stoichiometric ratio, for forming a ferroelectric material comprising SrBi2Ta2O9 from the metal oxide ceramic layer on the substrate, and the substrate on which the metal oxide ceramic layer is formed may be thereafter ferroannealed at an annealing temperature of about 650-820xc2x0 C. in oxygen or air at a pressure of about 1 atmosphere. Where the substrate comprises silicon containing a surface layer of a metal, e.g., Pt, after the substrate is ferroannealed, another layer of said metal is provided on the metal oxide ceramic layer, and the substrate is thereafter again ferroannealed at said temperature and pressure in oxygen or air, to form a capacitor having two layers of said metal as the top and bottom electrodes separated by the metal oxide ceramic layer as a dielectric.
The invention also contemplates the capacitor product made by the above described method.
According to one preferred embodiment, a method of forming a bismuth oxide ceramic layer on a substrate by CVD is provided, comprising conducting a gaseous flow of an oxygen-containing inert carrier gas comprising about 40-80% oxygen and a vaporized solution of a precursor organo bismuth compound in a volatile organic solvent, in the presence of a protonating additive substance in gaseous state comprising water, a volatile carboxylic acid compound, a volatile ketone, a volatile amine or ammonia, into contact with a surface portion of the substrate under a vacuum pressure of about 0.1-100 Torr at a thermal decomposition temperature of about 150-800xc2x0 C. to convert the precursor compound to its corresponding bismuth oxide, e.g., having the same oxidation state as in the precursor compound.
The additive substance is present in an amount sufficient to facilitate the thermal decomposition of the precursor compound and control the in situ oxidation state of the deposited metal and the amount of oxygen in the formed layer, e.g., while suppressing the formation of volatile intermediates and of vacancies in the formed layer, as noted above.
According to another preferred embodiment, a method of forming a bismuth oxide ceramic layer on a substrate by CVD is provided, comprising conducting a gaseous flow of an oxygen-containing inert carrier gas comprising about 40-80% oxygen and a vaporized solution of a mixture of precursor organo metal compounds comprising a precursor triorgano bismuth compound, a precursor diorgano strontium compound and a precursor pentaorgano tantalum compound in a volatile organic solvent, in the presence of a protonating additive substance in gaseous state comprising water, a carboxylic acid compound, a ketone, an amine or ammonia, as aforesaid, into contact with a surface portion of the substrate under a vacuum pressure of about 0.1-100 Torr at a thermal decomposition temperature of about 150-800xc2x0 C. to convert the triorgano bismuth compound to bismuth trioxide, the diorgano strontium compound to strontium monoxide and the pentaorgano tantalum compound to tantalum pentoxide.
The additive substance is present in an amount sufficient to facilitate the thermal decomposition of the triorgano bismuth compound, the diorgano strontium compound and the pentaorgano tantalum compound and to control the in situ oxidation state of the corresponding deposited metal and the amount of oxygen in the formed layer, while suppressing the formation of volatile intermediates and of vacancies in the formed layer.
According to further preferred embodiment, a method of forming a bismuth oxide ceramic layer on a substrate by CVD is provided, comprising conducting a gaseous flow of an oxygen-containing inert carrier gas comprising about 40-80% oxygen and a vaporized solution of a mixture of precursor organo metal compounds comprising bismuth tris-2,2,6,6-tetramethyl heptane-3,5-dionate or triphenyl bismuth, plus strontium bis-2,2,6,6-tetramethyl heptane-3,5-dionate and tantalum 2,2,6,6-tetramethyl heptane-3,5-dionate tetra isopropoxide, e.g., in a molar ratio of 0.25:0.15:0.40, in a volatile organic solvent comprising tetrahydrofuran/isopropyl alcohol/tetraglyme in a molar ratio of 8:2:1, or octane/decane/pentamethyl diethylene triamine in a molar ratio of 5:4:1, in the presence of a protonating additive substance in gaseous state comprising water, a carboxylic acid compound, a ketone, an amine or ammonia, as aforesaid, into contact with a surface portion of the substrate under a vacuum pressure of about 0.1-100 Torr at a thermal decomposition temperature of about 150-800xc2x0 C. for converting the bismuth tris-2,2,6,6-tetramethyl heptane-3,5-dionate or the bismuth tripenyl to bismuth trioxide, the strontium bis-2,2,6,6-tetramethyl heptane-3,5-dionate to strontium monoxide and the tantalum 2,2,6,6-tetramethyl heptane-3,5-dionate tetra isopropoxide to tantalum pentoxide.
The additive substance is present in an amount of about 0.01-200 mols per mol of each of the corresponding bismuth tris-2,2,6,6-tetramethyl heptane-3,5-dionate or bismuth triphenyl, plus strontium bis-2,2,6,6-tetramethyl heptane-3,5-dionate and tantalum 2,2,6,6-tetramethyl heptane-3,5-dionate tetra isopropoxide, for facilitating the thermal decomposition of the precursor organo compounds and for controlling the in situ oxidation state of the corresponding deposited metal and the amount of oxygen in the formed layer, e.g., while suppressing the formation of volatile intermediates and of vacancies in the formed layer.
According to a second aspect of the invention, a method of forming a metal oxide ceramic layer on a substrate by CVD is provided, comprising conducting a gaseous flow of a vaporized solution of a precursor organo metal compound in a volatile organic solvent, e.g., plus an oxidizing gas, in the presence of a protonating additive substance in gaseous state comprising a volatile lower alcohol having 1-4 carbon atoms, into contact with a surface portion of the substrate under a vacuum pressure at a thermal decomposition temperature below 450xc2x0 C., such as about 300-445xc2x0 C. in the case of a silicon substrate, or about 150-350xc2x0 C. in the case of a hydrocarbon based polymer substrate, for converting the precursor compound to its corresponding metal oxide, e.g., having the same oxidation state as in the precursor compound.
The alcohol additive substance is present in an amount sufficient for facilitating the thermal decomposition of the precursor compound and for controlling the in situ oxidation state of the deposited metal and the amount of oxygen in the formed layer, e.g., while suppressing the formation of volatile intermediates and of vacancies in the formed layer.
The alcohol additive substance may be present in an amount of about 25-200 mols per mol of the precursor compound. The vacuum pressure may be about 0.01-100 Torr, and especially about 0.5-20 Torr.
According to a third aspect of the invention, a method of forming a metal oxide ceramic layer on a substrate by CVD is provided, comprising conducting a gaseous flow of a vaporized solution of a precursor organo metal compound in a volatile organic solvent, e.g., plus an oxidizing gas, in the presence of an activating agent in gaseous state, into contact with a surface portion of the substrate under a vacuum pressure at a thermal decomposition temperature effective for converting the precursor compound to its corresponding metal oxide having the same oxidation state as in the precursor compound. The activating agent comprises carbon monoxide, hydrogen or a volatile lower aliphatic hydrocarbon having 1-4 carbon atoms or volatile aromatic hydrocarbon, which volatile hydrocarbon is optionally substituted by a bromo, iodo or nitro substituent, as aforesaid.
The activating agent is present in an amount sufficient for producing in situ active, especially hydrogen-active, compounds for enhancing the converting of the precursor compound to its said metal oxide and for controlling the in situ oxidation state of the deposited metal and the amount of oxygen in the formed layer, e.g., while suppressing the formation of volatile intermediates and of vacancies in the formed layer.
The decomposition temperature may be about 150-800xc2x0 C., especially below 450xc2x0 C., and the vacuum pressure may be about 0.01-100 Torr, preferably about 0.5-20 Torr.
The substrate may comprise silicon containing a surface layer of a metal, and the decomposition temperature may be about 300-500xc2x0 C. and the vacuum pressure may be about 0.1-100 Torr, preferably about 0.5-20 Torr. Alternatively, the substrate may comprise a hydrocarbon based polymer containing a surface layer of a metal, and the decomposition temperature may be about 150-350xc2x0 C. and the vacuum pressure may be about 0.1-100 Torr, preferably about 0.5-20 Torr.
The precursor compound typically comprises a mixture of bismuth tris-2,2,6,6-tetramethyl heptane-3,5-dionate or bismuth triphenyl, plus strontium bis-2,2,6,6-tetramethyl heptane-3,5-dionate and tantalum 2,2,6,6-tetramethyl heptane-3,5-dionate tetra isopropoxide, e.g., in a molar ratio of 0.25:0.15:0.40, respectively. The strontium bis-2,2,6,6-tetramethyl heptane-3,5-dionate may be provided in the form of the corresponding tetraglyme adduct or pentamethyl diethylene triamine adduct.
The mixture may be provided in a solvent system of tetrahydrofuran/isopropyl alcohol/tetraglyme, such as in a molar ratio of 8:2:1, respectively, or of octane/decane/pentamethyl diethylene triamine, such as in a molar ratio of 5:4:1, respectively.
The invention will be more readily understood from the following detailed description taken with the accompanying drawings and claims.